1. Field of the Invention
This, invention concerns an improved coating composition for use in preparing a coated cellulosic sheet material, for example paper or cardboard, which has special advantages when the coated sheet material is printed by the rotogravure process.
2. Description of the Background Art
Coating compositions for cellulosic sheet materials generally comprise an aqueous suspension of one or more pigments and one or more adhesive materials, together with small amounts of other additives such as rheology modifiers, lubricants etc, The most commonly used pigments are generally natural or synthetic inorganic materials of white colour and of relatively fine particle size distribution such that at least 45% by weight of the particles have an equivalent spherical diameter (esd) smaller than 2 .mu.m, when measured by a sedimentation method. Examples of pigments which are commonly used in coating compositions for cellulosic sheet materials are kaolin clay, calcined kaolin clay, natural or precipitated calcium carbonate, satin white, calcium sulphate and talc.
The rotogravure printing process is generally most suited to those applications in which a very large number of copies is required to be printed, because the process by which a rotogravure printing cylinder is prepared is expensive relative to the cost of preparing printing plates for use in other methods of printing. A rotogravure printing cylinder has on its surface a matrix of cells or depressions which vary in depth according to the amount of ink which is required to be transferred to the paper from each individual cell. The surface of the cylinder is initially smooth and highly polished, arid the matrix of cells on the surface is prepared by an expensive photographic etching process. Because of the large number of copies to be printed, it is generally desirable to run the printing press at high speed, and this necessitates the use of a printing ink which dries quickly on the paper to avoid "offset", or the transfer of undried printing ink from one sheet of paper to the next. It is therefore desirable to use a printing ink which has a solvent which is more volatile than water. In addition a solvent-based ink generally gives better print quality than a water-based ink. Such volatile solvents are almost always organic and oleophilic.
It has been discovered in the paper coating industry that, when large reels of paper are wound or unwound at high speed, frequent breakages can occur in the web of coated paper. One cause of this is believed to be the relatively high coefficient of friction of the coated surface of certain types of coated paper, which limits the freedom of movement of one turn of a reel of coated paper relative to an adjacent turn.
In order to reduce or eliminate this problem, some operators include in the coating composition from 10% to 45% by weight of talc in the total dry weight of the pigment in the composition. This reduces the coefficient of friction of the surface of the coated paper prepared using the composition and gives it a "slippery" feel. Talc of the required quality is, however, expensive, and in some cases, the slippery surface of the coated paper can cause problems in printing processes in which the paper is used.
It is also known (see for example TAPPI Monograph Series No. 25 "Paper Coating Additives", Chapter 2, "Lubricants" by R. B. Porter, Technical Association of the Pulp and Paper Industry, New York, 1963) to include in a paper coating composition as a lubricant or "levelling agent" a small quantity of a water-soluble soap, such as sodium or ammonium stearate, or of a soap which is insoluble, but dispersible, in water such as calcium or aluminium stearate.
Talc as used in the prior art to assist smoothness arid runnability is expensive.
Providing a paper having a surface which is compatible with the rotogravure printing process required to be operated has been difficult in the prior art. In many cases, there has been an incompatibility which has resulted in a loss of dots making up the required print image.
The present invention concerns the use of a surface treatment agent having hydrophilic and hydrophobic groups in the production of a coating composition for paper and like fibrous sheet materials. Use of such an agent in such compositions for different reasons is known from various prior art references, eg. GB 1,056,824; U.S. Pat. No. 4,908,240; U.S. Pat. No. 4,857,110; U.S. Pat. No. 4,766,015; U.S. Pat. No. 4,676,836; U.S. Pat. No. 3,928,707 and GB 674,876.
However, such references do not teach the production of compositions which solve both the `runnability` and `missing dot` problems described above by use of a surface treatment agent (without other measures).
EP-A-0565691 discloses a method whereby a pigment material is treated with a chemical surface treatment agent, e.g. a saturated or unsaturated fatty acid or an amine, prior to the preparation of the paper coating composition. However, in use of such a method, the treated pigment material has to be subsequently dispersed with dispersing agent in an aqueous suspension. Such a process is difficult to carry out satisfactorily because the coated pigment material is not easily dispersed with conventional dispersing agents.